Revert "fix build warning with arm64 gcc 5.4"

This reverts commit da3c206ebde6c29904fb46a61ec7534f90c0d08e.

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/common.h b/library/common.h
index ec30a7d..c20f6b2 100644
--- a/library/common.h
+++ b/library/common.h
@@ -165,10 +165,7 @@
  * \param   b Pointer to input (buffer of at least \p n bytes)
  * \param   n Number of bytes to process.
  */
-static inline void mbedtls_xor(unsigned char *r,
-                               const unsigned char *a,
-                               const unsigned char *b,
-                               size_t n)
+inline void mbedtls_xor(unsigned char *r, const unsigned char *a, const unsigned char *b, size_t n)
 {
     size_t i = 0;
 #if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS)